A lithographic printing plate generally comprises a hydrophobic imaging area, which receives oily ink in a printing process, and a hydrophilic non-imaging area, which receives dampening water. A conventional lithographic process usually comprises steps of masking a presensitized (PS) plate, which comprises a hydrophilic support and a hydrophobic photosensitive resin layer, with a lith film, exposing the plate to light through the lith film, and then developing the plate to remove a non-imaging area with a developing solution.
Nowadays a computer electronically processes, stores and outputs image information as digital data. A presensitized lithographic plate is preferably scanned directly with a highly directive active radiation such as a laser beam without use of a lith film to form an image according to a digital data. The term of Computer to Plate (CTP) means the lithographic process of forming a printing plate according to digital image data without use of a lith film.
The conventional lithographic process of forming a printing plate has a problem about CTP that a wavelength region of a laser beam does not match a spectral sensitivity of a photosensitive resin.
The conventional PS plate requires a step of dissolving and removing a non-imaging area (namely, developing step). The developed printing plate should be further subjected to post-treatments such as a washing treatment using water, a rinsing treatment using a solution of a surface-active agent, and a desensitizing treatment using a solution of gum arabic or a starch derivative. The additional wet treatments are disadvantageous to the conventional PS plate. Even if an early step (image-forming step) in a lithographic process is simplified according to a digital treatment, the late step (developing step) comprises such troublesome wet treatments that the process as a whole cannot be sufficiently simplified.
The printing industry as well as other industries is interested in protection of global environment. Wet treatments inevitably influence global environment. The wet treatments are preferably simplified, changed into dry treatments or omitted from a lithographic process to protect global environment.
A process without wet treatments is referred to as a press development method, which comprises the steps of attaching an exposed presensitized printing plate to a cylinder of a printer, and rotating the cylinder while supplying dampening water and ink to the plate to remove a non-imaging area from the plate. Immediately after exposing the presensitized plate to light, the plate can be installed in a printer. A lithographic process can be completed while conducting a usual printing treatment.
A presensitized lithographic printing plate suitable for the press development method must have a photosensitive layer soluble in dampening water or a solvent of ink. The presensitized plate should easily be treated under room-light to be subjected to a press development in a printer placed under room light.
A conventional PS plate cannot satisfy the above-described requirements.
Japanese Patent No. 2,938,397 (corresponding to European Patent No. 0770494, and U.S. Pat. Nos. 6,030,750 and 6,096,481) discloses a method for making a lithographic printing plate. The method uses an imaging element (presensitized plate) comprising on a hydrophilic surface of a lithographic based an image forming layer comprising hydrophobic thermoplastic polymer particles capable of coalescing under the influence of heat and dispersed in a hydrophilic binder and a compound capable of converting light to heat. The method comprising the steps of imagewise exposing to light the imaging element; and developing a thus obtained imagewise exposed imaging element by mounting it on a print cylinder of a printing press and supplying an aqueous dampening liquid or ink to the image forming layer while rotating the printer cylinder.
The imaging element can be treated under room light because the element has sensitivity within an infrared region.
In the method for making a lithographic printing plate, polymer particles coalesce under the influence of heat converted from light. Imaging elements having particles suitable for a press development often show poor plate wear.
Japanese Patent Publication Nos. 2000-211262, 2001-277740, 2002-29162, 2002-46361, 2002-137562 and 2002-326470 disclose presensitized lithographic printing plate in which microcapsules containing a polymerizable compound are dispersed in place of the thermoplastic polymer particles. An image formed by reaction of the polymerizable compound has stronger durability and gives better plate wear than an image made of the melted and aggregated particles.